Impedance matching is the practice of designing the input impedance of an electrical load (or the output impedance of its corresponding source) to maximize power transfer or minimize reflections. The maximum possible power is delivered to the load when the impedance of the load is equal to the complex conjugate of the impedance of the source.
Impedance matching can be provided by various components, including transformers, resistive networks, and filters. Communication systems typically require optimization of operation across a frequency band. Since it is not, in general, possible to achieve perfect impedance matching across multiple frequencies with discrete components, impedance matching networks designed with specific bandwidth most often take the form of a filter. Filters also provide the desired frequency discrimination needed in communication systems.
The electromagnetic spectrum is becoming increasingly crowded due to the proliferation of wireless communication systems. Commercial and military applications both face situations where performance of wireless communications systems is compromised by the proximity of multiple high power systems. The ability to insert a programmable low loss bandstop filter at the input of the receiver of a given system would go a long way toward alleviating the problems presented by high density signal environments.
In addition, wideband, reconfigurable efficient antenna matching at high tuning speeds and high power, particularly at low frequencies where the antennas used are much smaller than a wavelength, is challenging. Relays are limited by speed of operation and size, and solid state switches are fast but may not be able to withstand the high voltages associated with the high Q values encountered in matching small antennas. Also, solid state switches cannot be characterized as simple relays or knife switches because of their parasitic capacitances and power supply lines which affect the operation of high Q components.